In magnetic resonance imaging (“MRI”), the body of a subject to be imaged as, for example, the body of a medical patient, is subjected to a strong static magnetic field. Radio frequency (“RF”) excitation signals are applied to the subject. This causes the tissues of the subject's body to emit minuscule radio frequency signals referred to herein as “magnetic resonance signals.” During the procedure, magnetic field gradients are applied so that, during different portions of the procedure, the strength of the static magnetic field varies with distance along various axes. The resulting magnetic resonance signals are spatially encoded. Thus, the magnetic resonance signals typically are generated only in a limited region as, for example, a single point, a line or a two dimensional “slice.” Moreover, the signals from different portions of a line or a slice differ in frequency or phase from one another. If the procedure is repeated numerous times, it is possible, using known techniques, to recover an image data set having data elements, each representing one or more properties of the magnetic resonance signals generated within a single, small volume element or “voxel.” Because properties of magnetic resonance signals vary with the composition of the material generating the signal, the signals generated by different tissues within the body will differ from one another. Thus, data elements representing voxels in different tissues will have different values. Such a data set can be used, for example, to provide a visually perceptible image such as a screen display or a printed picture showing different tissues within the body with different brightness or color.
Magnetic resonance imaging offers numerous advantages over other imaging techniques such as conventional x-ray imaging, fluoroscopy and CAT x-ray scanning. For example, MRI is capable of showing soft tissues in extraordinary detail and is capable of displaying subtle anatomical differences. Moreover, MRI does not require exposure of the subject or medical personnel to ionizing radiation.
Many conventional magnetic resonance imaging instruments require that a patient lie on a horizontal bed that is then advanced into a tubular bore within a superconducting solenoidal magnet used to generate the static magnetic field. Other conventional MRI imaging instruments use a magnet having a ferromagnetic frame defining a patient-receiving space. Considerable effort has been devoted to design of such magnets in a manner which provides a relatively open patient-receiving space, as opposed to the claustrophobic tubular bore of the conventional solenoidal magnet. However, in these instruments as well, it has been the common practice to provide the patient on a bed which remains horizontal throughout the procedure.
It is often desirable to provide fixtures in close proximity to the patient. For example, local antennas such as small solenoidal coils can be placed around a part of the patient's body to be imaged as, for example, around the head or around a limb of the patient. These antennas can be used to transmit the RF excitation signals, to receive the magnetic resonance signals emitted by the tissue, or both. Such local antennas allow improved reception of signals from the specific region of interest within the patient's body. Other fixtures can be used for purposes such as supporting or positioning parts of the patient's body relative to the table as, for example, a rest for supporting the patient's head or limb. Typically, these fixtures are simply placed on the surface of the bed at the desired location for a particular patient, or are placed on the patient's body so that the fixture will be supported by the bed surface when the patient lies on the bed surface. These arrangements are satisfactory where the bed remains in a horizontal position at all times.
As described in greater detail in copending, commonly assigned U.S. patent application Ser. Nos. 08/978,084 and 09/718,946, the disclosures of which are hereby incorporated by reference herein, a magnetic resonance imaging system can be provided with a patient support such as a table which can extend in a generally vertical direction so that the long axis of the patient is substantially vertical. For example, the patient may be in a standing posture, with his back, side or front leaning against a generally vertical patient support. Such a support may include a footrest projecting from the table at its lower end and the patient may stand on the footrest. In other arrangements, the support includes a seat projecting from the table so that the seat is in a horizontal plane when the table surface is vertical. In particularly preferred arrangements, the patient support can move relative to the magnet. For example, the patient support may be arranged to move vertically relative to the magnet so as to elevate a portion of the patient into the patient-receiving space of the magnet. Alternatively or additionally, the patient support may be arranged to tilt through a range of orientations between a generally horizontal orientation and a generally vertical orientation.
Where the patient support table is in a generally vertical orientation during all or a portion of the procedure, fixtures positioned on the surface of the support will fall off of the support unless they are secured to the surface. Although the fixtures can be secured to the support using devices improvised for a particular application, as, for example, straps or tape, such arrangements do not offer a complete solution. Accordingly, there has been a need for improved apparatus for positioning fixtures in magnetic resonance apparatus, and for magnetic resonance apparatus incorporating such improved positioning apparatus.